1. Field of the Invention
The present invention relates to an optical amplifier apparatus and a controlling method thereof, and an optical transmission system using the optical amplifier apparatus, and it relates, in particular to the optical amplifier apparatus and the optical transmission system being suitable to be applied to a wavelength multiplexing optical transmission system.
2. Description of Related Art
In recent years, accompanying with requirement of a low cost optical system, an optical transmission system of so-called a wavelength multiplexing optical transmission system has been studied, in which a plurality of optical signals different in wavelength thereof are multiplexed to be transferred through a single optical transmission fiber.
On the other hand, an optical amplifier apparatus, since it has a wide range in wavelength of optical signal to be amplified therewith and it has an ability of amplifying with low noise, is suitable for use as an amplifier apparatus in the wavelength multiplexing optical transmission system. Optical fiber added with a rare-earth material or metal therein or a semiconductor amplifier, which can construct the optical amplifier apparatus, has a dependency on the wavelength in gain thereof, therefore, a difference is occurred in an optical output or in the gain, for each wavelength, after amplification therewith.
The above-mentioned difference in the wavelength is added up or integrated, in particular in multistage in-line amplification with the optical amplifier apparatuses, thereby increasing the difference in optical power for the each wavelength. As a result of this, a maximum transmission distance in a total system is restricted by deterioration in a S/N ratio of the optical wavelength having the lowest power among the multiplexed wavelengths. Accordingly, it is very important to provide an optical amplifier apparatus having an ensured characteristic of flatness, i.e., no difference in the optical output for every wavelength, in the gain thereof.
Therefore, as a conventional method, there has been already known a method, "Flattening of characteristic in collective amplification of multi-wavelengths with an optical fiber amplifier using a control of amplification factor of fiber", in Institute of Electronics, Information and Communication Engineers of Japan, Technical Paper OCS94-66, OPE94-88(1944-11), for example.
In the conventional method mentioned in the above, a characteristic curve of wavelength--optical power which is complex and variable in the shape thereof with respect to changes in an input power is made a constant or flat under a predetermined condition, thereby the characteristic curve in the gain is compensated under the predetermined condition.
Namely, an optical signal which is multiplexed with four waves in wavelengths of -11 dBm is inputted into an optical amplifier, and an optical output as a total of the amplified optical signal is monitored, wherein a fiber gain controller (it is called as "AFGC" hereinafter) for controlling fiber gain is used so as to make a level of that output a constant value. In this manner, the fiber gain can be controlled at the constant value of 12 dB, thereby minimizing the difference for each wavelength.
Or, by use of an automatic power controller (called as "APC" hereinafter) with an optical attenuator, optical loss is adjusted while maintaining the fiber gain at the constant value of 12 dB, thereby inhibiting the changes in spectrum of the fiber gain if the amplification factor of the in-line amplifier is changed.